Chromium-based aluminum-ceramic coating compositions have been well known and considered for decades as the industry standard for forming coatings which are highly corrosion-resistant and heat-resistant. U.S. Pat. No. 3,248,251 to Allen over forty years ago recognized and described the ability of aluminum-ceramic chromium-based coatings to exhibit resistance to corrosion, heat and abrasion while retaining adhesion and flexibility. Such attributes have continued to make aluminum-ceramic coatings widely used in a variety of applications. Today, these coatings are relied upon by original equipment manufacturers (OEM's) in the aircraft industry for protection of various aircraft engine components subject to high temperature and corrosive environments. Department of Defense (DoD) depot facilities also use aluminum-ceramic coatings as part of their manufacturing for critically needed items. Additionally, the automotive industry and various other industries routinely use aluminum-ceramic coatings as a high performance protective coating.
The conventional aluminum-ceramic coating typically consists of a chromate-phosphate binder that is filled with metallic aluminum powder. Upon curing, the binder forms a matrix that is embedded with the aluminum powder particles. The matrix provides mechanical integrity to the coating, while the chromate passivated aluminum pigment network imparts efficient corrosion protection. Burnishing Al-filled coating by dry grit or glass bead-blasting compresses the coating to render it conductive, galvanically active and sacrificial (i.e., cathodically protective) to all steels. SermeTel W® is recognized in the industry as the performance standard for these types of coatings. Depending on the particular application and service conditions, the coating can be used alone to provide adequate corrosion protection. Alternatively, the coating can be utilized as a part of an overlay system, serving as a basecoat that is sealed with top coatings and/or sealers. By sealing porosity and voids in the basecoat, the top coating provides additional barrier protection, thereby extending the corrosion protective action of the whole overlay system, as well as enhancing its other functional properties (e.g., such as smoothness, temperature resistance, etc.) and its visual appearance.
Notwithstanding the wide spread utility of aluminum-ceramic coatings, hexavalent chromium Cr(VI) has been identified as an environmentally hazardous material of concern. Consequently, it has been targeted for elimination in accordance with recent changes in the policies of the DoD, Air Force and various OEM's. The effect has created a need for Cr-free high performance coatings that can exhibit properties at least the same as that of the aluminum-ceramic coatings with Cr-based binders.
In response to the identification of hexavalent chromium Cr(VI) as an environmentally hazardous material, various Cr-free coatings have been investigated as potential replacement coatings. For instance, one alternative Cr-free coating considered is an aluminum ceramic basecoat layer having a phosphate-based binder. The coating when employed in conjunction with Cr(VI)-free top coating, provides application properties (e.g., thickness, roughness, galvanic activity) and performance (e.g., salt spray corrosion resistance, high temperature heat oxidation resistance, erosion resistance, mechanical properties) comparable to the benchmark coating systems with SermeTel W® basecoat. Furthermore, when used alone, these basecoats with a phosphate-based binder exhibited corrosion resistance when exposed up to 500 hours in the Salt Spray test per ASTM B117. However, as a basecoat, the coatings developed a red rust in the scribe and the field when subject to more prolonged testing of up to 1000 hrs. Another drawback of this approach stems from a significant interaction of aluminum particles with the phosphate binder in a water-based slurry in the absence of Cr(VI) species that have a passivating effect on aluminum metal. As a result of this adverse interaction of the aluminum particles with the phosphate binder, the basecoat slurry cannot be maintained as a “one-part” composition, in which all of the constituents can be mixed together into a single formulation, without one or more of the constituents adversely affecting other constituents of the composition. Rather, the slurry must be maintained in storage as a two-part slurry, in which the aluminum powder is maintained separate from the aqueous binder, until the point of use when the binder and Al can be mixed. However, the pot life of the mixed slurry is only about 8 hours, beyond which a rapid deterioration of the mixture is observed, that manifests itself in agglomeration of Al particles leading to a significant increase in the particle size. Although some specific modifications to the aluminum ceramic coatings employing phosphate-based binders can improve the pot life to over 24 hrs, the slurries must undesirably remain a two-part slurry to avoid the adverse interaction of the aluminum particles with the phosphate binder.
As another alternative, aluminum ceramic coatings with silicate-based binders have been considered. One type of Cr-free, silicate-based binder is generally described in US Patent Pub. No. 2006/0166014. However, basecoat performance appears to be dependent upon layer thickness, with increased coating thicknesses to at least 2 mils being required for sufficient corrosion resistant properties,
There is a continuing need for improved one-part Cr-free coatings which can provide improved mechanical and functional properties, including corrosion and heat resistance, at a reduced coating thickness.